I have the Onyx 1640i and use it with Pro Tools 8. It will serve as your interface, eq, pre's, faders, etc. with nothing else needed. The way I mix with it is quite complex, and it handles everything very well. While tracking, you can send your tracks into the DAW either pre or post eq with a push of a button. You can be recording 16 mics, into protools, and having 16 channels coming back to the mixer to monitor and mix, all at the same time. When your'e ready to mix, you can have 16 channels coming from your DAW to the board, and when you're ready to bounce back down to your DAW, you simply push a button and it takes your 16 track mix, and records it to a stereo track in the DAW. You can also send and return up to 6 different aux channels into your DAW during mix down, as well as 4 subgroup channels too. It's very, VERY flexible and works great for a nice project studio. I'm recording a hard rock metal band right now and loving it.
The way I mix on it is kinda complex, and is based on a lot of Michael Brauers techniques for mixing (Brauerize). I leave channels 1-2 on the board open as my "Stereo Mix Return," which also is my effects return too. Channels 3-16 are then used for all my tracks I will be mixing. I send them all out to the board for fader moves, pannning, and eq...I also do some eq and compression inside the DAW. I then bus all my drums and bass tracks to the stereo group bus 1-2 on the board, which sends into Pro Tools on a stereo submix bus for my compression and eq. I send my guitar tracks to the stereo group bus 3-4 on the board, which sends to a separate stereo submix in Pro Tools for bus compression and eq. These 2 stereo busses in Pro Tools (drum/bass bus and guitar bus) then are sent to another "Stereo Mix" bus in pro tools, which returns to channels 1-2 on the board...my stereo mix. This is assigned to main mix on the board. Vocals and other high end instruments like keys are bussed and compressed/equed in the same manner, but usually ITB and then come straight from PT to the board and assigned to main mix.
All effects are set up in PT on aux channels, and I send signal from the board via the 6 aux knobs to whatever effects bus I want, and then that is all bussed to the "Stereo Mix" bus and returned to channels 1-2 on the board. It's great.
Yes, it's complex, but I've been getting the best results ever doing this. It makes sense for me to mix this way, and I'm having so much fun. I'm sure you are confused now, but it shows you how flexible this board can be.